1. Field of the Invention
The invention relates to a heart monitor or a cardio-therapy device comprising a heart monitor, such as an implantable heart stimulator. Implantable heart stimulators include cardiac pacemakers, cardioverters/defibrillators and the like. Heart monitors include implantable devices such as physiologic recorders, loop recorders, or implantable “Holter” recorders. The invention further refers to a method of processing electrogram signals.
2. Description of the Related Art
A primary task of an implanted cardiotherapy device is the classification of signals—usually electrogram (ECG) signals—indicating the electrical activity of the human heart. More particularly, a task for a heart monitor is the accurate identification of rhythm categories. The basis of intracardiac electrogram classifiers in presently implanted devices is comparing the signal amplitude to a threshold and comparing the frequency of threshold crossings to a rate limit.
In these systems, signal noise can be a problem, since noise may be mistaken as a cardiac event and, thus, may lead to misclassification of an electrogram signal as representing tachycardia.
Implantable heart stimulators can be used for treating a variety of heart disorders like bradycardia, tachycardia or fibrillation by way of electric stimulation pulses delivered to the heart tissue, the myocardium. Tachycardia is a phenomenon where the heart exhibits a heart rate that is faster than it should be and therefore is subject to therapy. Tachycardia ventricle can be a “slow” tachycardia that is called ventricular tachycardia (VT). There is another form of tachycardia called ventricular fibrillation (VF), where the ventricle exhibits a very high rate of contractions that are uncoordinated and, thus, seriously affect the ventricle's capability to pump blood. VF therefore can be lethal if not treated quickly, e.g. by means of a defibrillator delivering a defibrillation shock.
In any case, reliable assessment of the heart rate is needed. The heart rate is the rate with which natural contractions of a heart chamber occur.
In order to be able to sense a contraction, a heart chamber excitation that naturally occurs without artificial stimulation and that is called intrinsic, the heart stimulator usually comprises at least one sensing stage that is connected to a sensing electrode on said electrode placed in the heart chamber. Electrical potentials can be picked up via the sensing electrode and can be evaluated by the sensing stage in order to determine whether an intrinsic excitation—called: intrinsic event—has occurred.
Usually, heart stimulators feature separate sensing stages for each heart chamber of interest. In a dual-chamber pacemaker, usually two separate sensing stages, an atrial sensing stage and a ventricular sensing stage, are provided that are capable to detect intrinsic atrial events AS (atrial sensed event) or intrinsic ventricular events VS (ventricular sensed event), respectively.
As known in the art, additional separate sensing and pacing stages are provided for three-chamber (right atrium RA, right ventricle RV, left ventricle LV) or four-chamber (right atrium RA, left atrium LA, right ventricle RV, left ventricle LV) pacemakers or ICDs.
In a heart cycle, an excitation of the myocardium leads to depolarization of the myocardium that causes a contraction of the heart chamber. Thereafter, the myocardium repolarizes and thus relaxes and the heart chamber expands. In a typical electrogram (EGM), depolarization of the ventricle corresponds to a signal known as “R-wave”. The repolarization of the ventricular myocardium coincides with a signal known as “T-wave”. Atrial depolarization is manifested by a signal known as “P-wave”.
A natural contraction of a heart chamber thus can be detected by evaluating electrical signals sensed by the sensing channels. In the sensed electrical signal, the depolarization of atrium muscle tissue is manifested by occurrence of a P-wave. Similarly, the depolarization of ventricular muscle tissue is manifested by the occurrence of a R-wave. A P-wave or an R-wave thus leads to an atrial sense event AS or a ventricular sense event VS, respectively.
A R-wave or a P-wave usually is detected when a respective electrogram signal exceeds a predetermined threshold. Noise in that signal may lead to over-sensing that is sensing of “false” cardiac events that actually are nothing but noise in the signal.
A published article in the technical literature presented techniques to mitigate noise in subcutaneous ECG signals collected using an implantable recorder (Brignole et al, Journal of Cardiovascular Electrophysiology, Apr. 10, 2008). Simple detection schemes are outlined that are limited in their performance.
Solutions presented in above-cited article are susceptible to over-sensing of noise and disregarding portions of data that could otherwise be used for detection of arrhythmia (false positives for noise detection). If more than two refractory senses are detected, the device blanks until the next “true” cardiac event is detected. For detection of ECG during periods of muscular activity or excessive motion, the device could detect only noise for long periods of time and would subsequently be unable to detect arrhythmias occurring during this time.
Other methods are typically constrained by the available hardware, which is itself constrained by geometry and power. As a result, simple electronics are typically used in these devices, which do not provide extreme flexibility of signal processing and manipulation.